The pipes for transporting hydrocarbons are composed of pipe sections joined together to cover overall lengths in the order of hundreds of kilometres. The pipe sections have a unit length, generally 12 meters, and relatively large diameters of between 0.2 meters and 1.5 meters. Each pipe section comprises a steel cylinder, a coating in a polymer material that has the function of protecting the steel pipe, and a possible further coating in Gunite or concrete, the function of which is to weigh down the pipeline for underwater applications, or a further coating in an insulating material to thermally insulate the pipeline.
To enable welding the steel cylinders together, the opposite free ends of each pipe section have no coating. The pipe sections are joined to one another both in land installations to form lengths of pipe that are multiples of the unit length, and in laying vessels, where the pipe sections, of unit length or multiples thereof, are joined to lengths of pipe already joined to other lengths of pipe to form part of the underwater pipeline.
The joining operations envision welding the steel cylinders, usually in several welding passes, and restoring the coating.
Once a welding bead ring has been completed between two steel cylinders, an uncoated cutback straddles the weld. The cutback is basically the annular joint portion defined by the free ends of the pipe sections and extending axially between two end edges of the coating, and must be covered by a protective coating.
Application of the protective coating on the cutback is known as field joint coating and, in general, includes coating the cutback with three layers designed to ensure protection of the steel cylinders and the adhesion of these layers thereto. Field joint coating on the cutback includes: heating, by induction for example, the cutback up to a temperature in the range 160° C.-250° C.; coating the cutback with a fusion bonded epoxy (FBE) resin that forms, on contact with the cutback, a first, relatively thin layer, known as the primer; spraying on the cutback, on top of the primer, a modified copolymer with an adhesive function that forms, in contact with the primer, a second relatively thin layer; and applying a third coat, generally known as the top coat.
In general, the first layer, or primer, is applied by a cylindrical inductor, which is placed around the cutback and structured like a clamp, and an applicator that is movable with respect to the outer surface of the cutback to apply a polymer material on the cutback. Some examples of apparatuses for coating pipelines that enable heating the pipeline with a cylindrical inductor and applying the first layer in quick succession are described in PCT Patent Application No. WO 2009/024,755 and U.S. Pat. No. 4,595,607. Certain known apparatuses of the above-indicated type suffer from the drawback of not being adaptable to cutbacks of different length and, in consequence, for design reasons it is often necessary to have a plurality of coating apparatuses. PCT Patent Application No. WO 2013/115646 and PCT Patent Application No. 2011/162747 disclose apparatuses, which control applicators for applying coating to pipelines. However, these documents fail teaching how to heat the pipeline.